1. Field of the Invention
This invention pertains generally to the field of oven heatable plastic coated paperboard containers and to processes for producing the same.
2. Description of the Prior Art
The most common containers for convenience foods which are to be heated within the container are formed of thin sheet aluminum or layers which include aluminum foil. Because of the relative high cost of such containers and because they generally cannot be used in microwave oven cooking, substantial efforts have been made to provide plastic coated paperboard cartons which can withstand oven heating.
Polyethylene is often used as a coating material for paperboard since it has good moisture impermeability and is easily adhered to many types of paperboard. However, polyethylene and many other types of common plastic coating materials do not have the resistance to melting at high temperatures required for very hot oven heating. Such coating polymers must also have adequate structural strength and abrasion resistance, as well as being compatible with food products.
Polyethylene terephthalate polyester is a particularly satisfactory coating material for oven heatable trays since it has a high melting temperature and good structural strength, and is compatible with and unaffected by most food products. However, it is well known in the art that it is difficult to obtain good bonding of polyethylene terephthalate to other materials and particularly to paperboard. In the past, such bonding has been accomplished by the use of adhesives or primers applied over the paperboard before a hot melt extrusion of the polymer is applied to the paperboard. The use of primers and adhesives is undesirable in packaging foods because such materials are capable of migrating into the contents of the food package.
A procedure for extrusion coating polyethylene terephthalate onto paperboard without the use of primers is shown in U.S. Pat. No. 3,924,013 to Kane, in which the paperboard is subjected to heating prior to being contacted with the hot melt extrusion. While such a process may be adequate for certain purposes, it is undesirable were the coated paperboard is to be die pressed into deep formed trays, since heating the paperboard reduces its moisture content and embrittles the board to thereby make it more subject to tearing upon die pressing. Deep pressed heatable containers are especially preferred since they do not require the use of adhesives or heat seals in order to form the edge walls of the tray. Trays formed by adhesively connecting the sides of the tray together or by heat sealing them together are subject to separation at the very high temperatures of oven heating, and the adhesive material may migrate into the food product. Pressing allows formation of smooth radius contoured corners, rather than sharp adhesively joined corners, which provides good heat distribution characteristics during oven heating.